This invention relates to a catalytic process for the hydroconversion of heavy hydrocarbon streams containing asphaltenic material, metals, and sulfur compounds. More particularly, this invention relates to hydroconversion using a series of ebullated bed reactors containing a catalyst having improved effectiveness and activity maintenance in the desulfurization and demetallation of metal-containing heavy hydrocarbon streams.
As refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for processes to treat the fractions containing increasingly higher levels of metals, asphaltenes, and sulfur.
It is widely known that various organometallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbon streams derived from tar sands, and hydrocarbon streams derived from coals. The most common metals found in such hydrocarbon streams are nickel, vanadium, and iron. Such metals are very harmful to various petroleum refining operations, such as hydrocracking, hydrodesulfurization, and catalytic cracking. The metals and asphaltenes cause interstitial plugging of the catalyst bed and reduced catalyst life. The various metal deposits on a catalyst tend to poison or deactivate the catalyst. Moreover, the asphaltenes tend to reduce the susceptibility of the hydrocarbons to desulfurization. If a catalyst, such as a desulfurization catalyst or a fluidized cracking catalyst, is exposed to a hydrocarbon fraction that contains metals and asphaltenes, the catalyst will become deactivated rapidly and will be subject to premature replacement.
Although processes for the hydrotreating of heavy hydrocarbon streams, including but not limited to heavy crudes, reduced crudes, and petroleum hydrocarbon residua, are known, the use of fixed-bed catalytic processes to convert such feedstocks without appreciable asphaltene precipitation and reactor plugging and with effective removal of metals and other contaminants, such as sulfur compounds and nitrogen compounds, are not common because the catalysts employed have not generally been capable of maintaining activity and performance.
Thus, the subject hydroconversion processes are most effectively carried out in an ebullated bed system. In an ebullated bed, preheated hydrogen and resid enter the bottom of a reactor wherein the upward flow of resid plus an internal recycle suspend the catalyst particles in the liquid phase. Recent developments involved the use of a powdered catalyst which can be suspended without the need for a liquid recycle. In this system, part of the catalyst is continuously or intermittently removed in a series of cyclones and fresh catalyst is added to maintain activity. Roughly about 1 wt.% of the catalyst inventory is replaced each day in an ebullated bed system. Thus, the overall system activity is the weighted average activity of catalyst varying from fresh to very old i.e., deactivated.
Hopkins et al. in U.S. Pat. No. 4,119,531 disclose a process for hydrodemetallation of hydrocarbon streams containing asphaltenes and a substantial amount of metals, which comprises contacting the hydrocarbon stream with a catalyst consisting essentially of a small amount of a single hydrogenation metal from Group VIB or Group VIII, deposed on a large pore alumina; suitable examples of the hydrogenation metal are nickel or molybdenum. The catalyst is characterized by a surface area of at least 120 m.sup.2 /gm; a pore volume of at least 0.7 cc/gm and an average pore diameter of at least 125 .ANG. units.
Hensley et al. in U.S. Pat. No. 4,297,242 disclose a multiple-stage catalytic process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and a substantial amount of metals. The first stage of this process comprises contacting the feedstock in a first reaction zone with hydrogen and a demetallation catalyst comprising hydrogenation metal selected from Group VIB and/or Group VIII deposed on a large-pore, high surface area inorganic oxide support; the second stage of the process comprises contacting the effluent from the first reaction zone with a catalyst consisting essentially of hydrogenation metal selected from Group VIB deposed on a smaller pore, catalytically active support comprising alumina, said second stage catalyst having a surface area within the range of about 150 m.sup.2 /gm to about 300 m.sup.2 /gm, an average pore diameter within the range of about 90 .ANG. to about 160 .ANG., and the catalyst has a pore volume within the range of about 0.4 cc/gm to about 0.9 cc/gm. Hensley et al. disclose that as little as 2.2 wt.% cobalt oxide caused more rapid deactivation of their second-stage catalyst for sulfur removal.
In U.S. Pat. No. 4,212,729 to Hensley et al., another two-stage catalytic process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and metals is disclosed. In this process, the first-stage demetallation catalyst comprises a metal selected from Group VIB and from Group VIII deposed on a large-pore, high surface area inorganic oxide support. The second stage catalyst contains a hydrogenation metal selected from Group VIB deposed on a smaller pore catalytically active support having the majority of its pore volume in pore diameters within the range of about 80 .ANG. to about 130 .ANG..
Other examples of multiple-stage catalytic processes for hydrotreatment of heavy hydrocarbon streams containing metals are disclosed in U.S. Pat. Nos. 3,180,820 (Gleim et al., 1965); 3,730,879 (Christman, 1973); 3,977,961 (Hamner, 1976); 3,985,684 (Arey, et al., 1977); 4,016,067 (Fischer, 1977); 4,054,508 (Milstein, 1977); 4,051,021 (Hamner, 1977); and 4,073,718 (Hamner, 1978).
The catalysts disclosed in these references contain hydrogenating components comprising one or more metals from Group VIB and/or Group VIII on high surface area support such as alumina, and such combinations of metals as cobalt and molybdenum, nickel and molybdenum, nickel and tungsten, and cobalt, nickel, and molybdenum have been found useful. Generally, cobalt and molybdenum have been preferred metals in the catalysts disclosed for hydrotreatment of heavy hydrocarbon streams, both in first-stage catalytic treatment to primarily remove the bulk of the metal contaminants, and in second-stage catalytic treatment primarily for desulfurization.
A difficulty which arises in resid hydroprocessing units employing the above catalyst systems is the formation of insoluble carbonaceous substances also known as Shell hot filtration solids. These substances cause operability problems in the hydrotreating units. Certain resids tend to produce greater amounts of solids thereby limiting the level of upgrading by the amount of these solids the hydroprocessing unit can tolerate.
Further, the higher the conversion level for given feedstocks, the greater the amount of solids formed. In high concentrations, these solids accumulate in lines and separators, causing fouling, and in some cases interruption or loss of process flow.
Accordingly, it is a general object of this invention to provide a process affording a higher conversion level for heavy hydrocarbon feedstocks that tend to form greater amounts of insoluble substances, especially that fraction of the feedstock that boils over 1,000.degree. F.
It is another object of the present invention to provide a process that can tolerate larger amounts of insoluble carbonaceous substance producing feedstocks in the feed stream to the process.
It is yet another object of the present invention to provide a process that employs a less expensive catalyst system than conventional processes designed to handle heavy hydrocarbon feed streams and affords a longer catalyst life, i.e., stability.
These objectives can be attained by the process of the present invention which utilizes a novel catalyst to effect the hydroconversion of heavy hydrocarbon streams in a series of ebullated bed reaction zones. It has been discovered that the addition of a relatively small amount of cobalt to a high-macropore molybdenum-containing demetallation catalyst yields a single catalyst system, when utilized in an ebullated bed reactor, that provides the requisite demetallation, low solids formation, desulfurization and Ramsbottom carbon conversion.
In the two-stage prior art processes, such as those disclosed in U.S. Pat. Nos. 4,297,242, and 4,212,729, the demetallation catalyst is followed by a smaller-pore hydrotreating catalyst to ensure high-product quality. In the process of the present invention, it has been surprisingly discovered that the addition of a relatively small amount of cobalt to a high-macropore volume demetallation catalyst provides all of the above benefits in an ebullated bed reactor system. Further, it has discovered the addition of other metal promoters or a relatively greater amount of cobalt to the high-macropore demetallation catalyst does not increase the activity of the catalyst. Thus, the economics of the process of the invention are enhanced by the unexpected reduced need for expensive cobalt in the catalyst system.